1. Field of the Invention
The present invention relates to an oil injection type compressor including means for protecting the compressor when there occurs abnormality in the pressure of lubricating oil.
2. Prior Art of the Invention
There has been well known such an oil injection type screw compressor, as shown in FIG. 2, which includes an oil passage 18 for circulating lubricating oil. The oil passage 18 extends from an oil separator 13 disposed in a discharge passage 12 of a compressor main body 11 through an oil cooler 14, oil filter 15 and oil pump 16 disposed on the downstream side from the oil filter 15 to parts 11a requiring lubricating oil such as rotors, bearings, shaft sealing members within the compressor main body 11.
The lubricating oil thus supplied to the parts requiring lubricating oil is discharged together with compressed gas from the compressor main body 11 to the discharge passage 12, and is then separated from the compressed gas by the oil separator 13. The separated compressed gas is fed to the discharge passage 12 extending from the upper portion of the oil separator 13. Meanwhile the separated lubricating oil is cooled by the oil cooler 14, being filtrated by the oil filter 15, and is then supplied by the oil pump 16 or bypassing the oil pump 16 through a check valve 17 to the parts requiring lubricating oil, to be thus circulated thereafter. As for the oil pump 16, it is operated only when oil is initially supplied before starting the compressor main body 11 and also the supplied oil is decreased in its differential pressure during the ordinary operation. In other words, it is not operated during the normal operation.
Also, the conventional compressor is provided with first, second and third differential pressure switches 20, 21 and 22. The first differential pressure switch 20 functions to detect a pressure in an oil passage 18(a) between the oil separator 13 and oil cooler 14 as a lower pressure side, and a pressure of an oil passage 18(b) between the oil pump 16 and the compressor main body 11 as a higher pressure side. Switch 20 to thereby drives the compressor main body 11 when the detected differential pressure is not less than the specified value. The second differential pressure switch 21 functions to detect a pressure in a suction passage 19 as a lower pressure side, and a pressure in the oil passage 18(b) as a higher pressure side, to thereby automatically start the oil pump 16 when the detected differential pressure is not more than the specified value. The third differential pressure switch 22 functions to stop the compressor main body 11 when the detected differential pressure is furthermore decreased.
With the supplied pressure thus secured, the compressor main body 11 is started. In this case, when the above differential pressure becomes abnormally small due to increase in a suction pressure, decrease in a discharge pressure and blocking of the oil filter 15, the second differential pressure switch 21 is operated to start the oil pump 16. When the above differential pressure becomes furthermore abnormally small, the third differential pressure, switch 22 is operated to cause an emergency stop of the compressor main body 11. Accordingly, the compressor main body 11 can secure the differential pressure of the supplied oil.
As mentioned above, the conventional compressor is so constructed as to detect the differential pressure between a pressure of the suction passage 19 and a pressure of the oil passage 18(b), and hence to start or manually stop the oil pump 16 based on the detected differential pressure. Consequently, when the oil pump 16 is stopped, the pressure on the discharge side of the oil pump 16 is decreased due to its stoppage, and hence the differential pressure is made smaller as a result of which the oil pump 16 will be re-started. Namely, the above differential pressure is effected by the starting and stoppage of the oil pump 16.
Therefore, the conventional compressor is disadvantageous in that it is required to manually operate the oil pump 16 after confirming the enough recovery of the differential pressure of the supplied oil.
In order to solve the above disadvantage, it may be considered that the switching difference between from on to off and from off to on in the second differential pressure switch 21 is made larger than the pump boosting rate, or one more differential pressure switch is added.
However, the above switching difference cannot be arbitrarily selected and is difficult to be made larger. Also, the addition of the differential pressure switch makes the pressure detecting system complex and expensive.